a) Field of the Invention
This invention relates to ion-conductive polymers, in other words, to polymeric solid electrolytes and also to a process for the production thereof.
b) Description of the Related Art
Development of organic polymeric solid electrolytes is in progress in recent years, which compared to inorganic solid electrolytes, have such merits that (1) they have formability and can be easily formed into thin films of a large area and (2) they have flexibility and hence excellent contiguity with electrodes.
Mixtures of polyethylene oxide and alkali metal salts were proposed as polymeric solid electrolytes by M. B. Armand et al ["Fast Ion Transport in Solids" 131: North Holland Publishing Co., Ltd., 1979]
These solid electrolytes are each formed into a film by dissolving it in a solvent, casting the resulting solution on a forming surface and then drying off the solvent, namely, by the so-called casting process. Their forming process is therefore complex, and the films so obtained have an electrical conductivity as low as 10.sup.-6 S/cm or less at room temperature and fail to exhibit satisfactory contiguity with electrodes. There has accordingly been a desire for the improvements of these drawbacks.
Other production processes include the process disclosed in Japanese Patent Laid-Open No. 48716/1987 in which crosslinking is induced by a reaction between a trifunctional polyethylene glycol and a diisocyanate derivative; and the process disclosed in Japanese Patent Laid-Open No. 285954/1987 in which crosslinking is induced by a polymerization reaction of polyethylene glycol diacrylate. Each of these processes however uses a solvent so that an extra step is indispensable for drying off the solvent. Further improvements have also been considered necessary with respect to the balancing among the ionic conductivity, the contiguity with electrodes and so on.
In addition, the ionic conductivity of a polymeric solid electrolyte drops significantly in a low temperature range below room temperature. There has also been a strong demand for an improvement in this respect.
The present inventors have already proposed a process for the production of a polymeric solid electrolyte of the two-pack mixed hardening type, which process has overcome the above-described drawbacks (Japanese Patent Application No. 62685/1990).
Further improvements are however desired from the standpoint of the ionic conductivity of the polymeric solid electrolyte per se and the contiguity with electrodes. Described specifically, there has been a strong demand for a polymeric solid electrolyte whose ionic conductivity is 10.sup.-4 S/cm or higher at 0.degree. C.